Para enviar para um usuário, dê uma olhada no comando write . Na página do manual relevante:
The write utility allows you to communicate with other users, by copying lines from your terminal to theirs. When you run the write command, the user you are writing to gets a mes‐ sage of the form: Message from yourname@yourhost on yourtty at hh:mm ... Any further lines you enter will be copied to the specified user's termi‐ nal. If the other user wants to reply, they must run write as well. When you are done, type an end-of-file or interrupt character. The other user will see the message ‘EOF’ indicating that the conversation is over.
Para enviar para vários usuários, veja o comando wall :
Wall displays the contents of file or, by default, its standard input, on the terminals of all currently logged in users. The command will cut over 79 character long lines to new lines. Short lines are white space padded to have 79 characters. The command will always put carriage return and new line at the end of each line. Only the super-user can write on the terminals of users who have chosen to deny messages or are using a program which automatically denies mes‐ sages.
Também relevante é o comando mesg :
Mesg controls the access to your terminal by others. It's typically used to allow or disallow other users to write to your terminal (see write(1)).